(1) Field of the Invention
The present invention relates to a double-base propellant, and more particularly to a double-base propellant for artillery and small arms, which consists mainly of nitroglycerine and nitrocellulose crosslinked three-dimensionally with a polyfunctional isocyanate, and a method of producing the same.
(2) Description of the Prior Art
Granular propellants for artillery and small arms are generally desired to have such a shooting performance that the propellants have a high muzzle velocity at a low bore pressure. An effective means for increasing a muzzle velocity in a double-base propellant is to increase the amount of nitroglycerine contained therein as one of the main components.
However, in the conventional double-base propellant compositions, the mixing ratio of nitroglycerine to nitrocellulose used as main components thereof is substantially constant, and the amount of nitroglycerine contained in the double-base propellant composition is generally suppressed to as low as less than 30% by weight. One of the reasons is as follows. The use of a small amount of nitrocellulose and a large amount of nitroglycerine in the production of double-base propellant is advantageous in Japan due to its low production cost. However, when a double-base propellant containing a large amount of nitroglycerine is produced by a conventional extrusion forming process, the double-base propellant composition is soft due to its high nitroglycerine content, and the resulting granules produced by the extruding the composition and cutting the extruded composition aggregate mutually to be easily formed into a block, and are poor in the fluidity. Therefore, the final product of granular propellant cannot be easily charged into the cartridge case. Another reason is as follows. Granular propellant containing a large amount nitroglycerine are apt to have an extraordinarily high bore pressure at the shooting, and have a problem in the public security.
One of the method of solving the above described high bore pressure is disclosed in Japanese Patent Laid Open Application No. 48,409/76. In this method, the surface of granules of single-base propellant consisting mainly of nitrocellulose is covered with a restrictor consisting of a reaction product of a polyfunctional isocyanate and a polyol, whereby the combustion of the propellant is suppressed to prolong the time until the pressure generated at the combustion of the granules of the propellant reaches the highest pressure. However, in this method, it is difficult to regulate the thickness of the above described restrictor covering the surface of the propellant granules, and therefore the propellant having the coating is apt to be uneven in the combustion performance and further is low in the ignitability. Moreover, this method must be carried out by additionally adding one step to the steps in the conventional production method, and the production of a propellant by this method is expensive. The inventors have studied to obviate the above described drawbacks and accomplished the present invention.